Synthesis and Rheological Properties of Branched Polylactide

Abstract

Plastics are essential to society but the current trends of their production, use, and end-of-use are not sustainable. This realization has created a push towards more circular approaches to plastics management and central to this is the transition away from petroleum-based feedstocks towards those that are biobased and inherently renewable. This shift not only relies on the development of new biobased polymers but also the expanded use of those currently available. Polylactide has found its place as a key material in the biobased plastics market and is projected to remain so for the foreseeable future. However, its use is hampered by its poor melt processability in extensional flows. The work in this dissertation has sought to better understand how the polymer architecture can improve this limitation. The research presented here describes the means of implementing PLA into an architecture with precision branching and the study of how controlled changes to the branching influences the melt flow behavior. The introduction to this dissertation overviews general concepts of extensional rheology and branched polymer dynamics, which are important to the research. Chapter 2 discusses how to create graft polymers of PLA and focuses on the effects of monomer size and feed composition on the copolymerization kinetics for a graft-through synthesis. Chapter 3 uses this chemistry to synthesize a library of model graft copolymers to study how changes to the architecture influence viscoelasticity in extensional and shear deformations. In Chapter 4, a new method for synthesizing H-shaped PLA homopolymers is presented and the associated rheological properties are compared to a linear analogue. Taken together, these works further the ability to synthesize polymers with controlled branching and broadens the understanding of how specific changes to the branching can influence the rheological melt behavior. These ideas can be leveraged to target materials with viscoelastic properties amenable to industrial processing flows so that they can be used for a broader variety of applications.